1. Field of the Invention
In a fibre-optic network, the wavelength of light used to communicate a signal is an important parameter. In particular, where Dense Wavelength Division Multiplexing (DWDM) systems are employed, different signals are communicated using respectively different wavelengths. Consequently, it is important to maintain the different wavelengths accurately in relation to components of the DWDM system, for example multiplexers and demultiplexers, which add or remove wavelengths from the WDM system.
2. Brief Description of Related Developments
Typically, a semiconductor laser device is employed in an optoelectronic transmitter unit of a DWDM fibre-optic network. In order to maintain the output of the laser at the desired wavelength, the temperature of the laser must be accurately controlled. Thus, if the ambient temperature of the air surrounding an optoelectronic module is lower than the operating temperature of the laser, the laser must be heated up to its operating temperature and, conversely, if the ambient temperature of the air surrounding the optoelectronic module is higher than the operating temperature of the laser, the laser must be cooled down.
If the ambient air temperature can be guaranteed to remain below a particular level, then the laser temperature can be relatively easily controlled without overheating the module. However, in many circumstances, the module forms part of a larger system with many other components generating heat in the environment, so that the ambient air temperature may be much higher. In any event, the module must be able to operate at a specified ambient temperature range, which may reach as high as 75° C.
It will be appreciated that an optoelectronic module will dissipate heat if the ambient air temperature surrounding the module is lower than the temperature of the module casing. Of course, the greater the temperature differential between the module casing and the surroundings, the greater the rate at which heat will be dissipated from the module casing. Fins can be used to increase the rate of heat dissipation by increasing the surface area of the casing, but the provision of fins increases the external size of the module, which may be unacceptable in some circumstances, or reduces the internal space in the module, which may be similarly unacceptable. Furthermore, as the size of optoelectronic modules decreases with rising miniaturisation, the internal components in the module are packed more tightly together and the surface area of the module is reduced, in both cases exacerbating the problem of heating in the module.
Therefore, in many optoelectronic modules including a laser, a thermoelectric device has been used to control the temperature of the laser. For example, U.S. Pat. No. 6,247,524 teaches that the wavelength emitted by the laser diode in a diode-pumped solid state laser is extremely temperature-sensitive and therefore requires a temperature control mechanism to stabilise the diode at a set operating temperature. At low ambient temperatures in the “stand-by” mode, the diode laser is heated in order to stabilise the laser at the setpoint temperature. The thermoelectric device can heat the laser if the ambient temperature surrounding the module is low, but can also cool the laser if the ambient temperature surrounding the module is high relative to the operating temperature of the laser. Typical figures are that the operating temperature of the laser is about 25° C., with a module case temperature of about 75° C. It will be appreciated that this requires the laser to be cooled, which utilises substantial amounts of electrical power and consequently dissipates far more heat than it removes from the laser.